In the absence of gravity, the weight of the astronaut and that of the objects disappear completely. This induces modifications of the bones and the muscles which can severely affect the astronaut’s life during the flight, and even more when returning to earth. This is a fundamental point to study, since it may become a limiting factor and even an obstacle to long duration flights. This experiment should also permit the test of physical exercise program and of a drug, pamidronate (AREDIA®), as counter-measures against the effects of weightlessness on the locomotion system (skeleton and muscles).